In general, a narrow viewing angle is one of the most vulnerable points of a liquid crystal display having good points in terms of lightweight, thin type, low power consumption and the like. The narrow viewing angle means that a completely different image is displayed on a display screen depending on a direction to which a person sees the display screen. As such, the reason why an image is differently seen depending on a viewing angle in a liquid crystal display results from the anisotropy of liquid crystal and imperfection of a polarizing plate.
The viewing angle refers to an angle at which a contrast ratio is usually 10:1 or more. To this end, a dark state being a completely dark state and uniform brightness are required. A variety of attempts for improving a viewing angle of a liquid crystal display have been made considering such requirements.
As an example of methods for improving an optical viewing angle, in TN (Twisted Nematic) mode is used a method such as a compensation film, DDTN (Dual-Domain TN), MDTN (Multi-Domain TN) or the like. In VA (Vertical Alignment) mode, for example, is used an advantage in that a normally black mode in which an initial alignment direction is set to be a vertical direction is used contrary to a TN mode liquid crystal display frequently using a normally white mode so that a complete dark state is achieved due to a dark state by means of two sheets of front and rear polarizing plates, thereby increasing a contrast ratio. In IPS (In-Plane Switching) mode is used a method in that liquid crystal is rotated on a plane when the liquid crystal is horizontally aligned, and two electrodes are driven on one substrate so that light is transmitted or shielded, thereby improving an optical viewing angle.
In particular, since light quantity is controlled by means of horizontal movement of liquid crystal in the IPS mode so that a viewing angle characteristic becomes considerably better, the IPS mode is being used in a large number of products at present. Contrary to the VA mode, the IPS mode has superior characteristics of a viewing angle and a color shift even in a state where a retardation film is not basically applied.
Further, U.S. Pat. No. 6,115,095 discloses that there are suggested structures where +C-PLATE, which is a positive uniaxial, optical anisotropic film and perpendicular to a plane with respect to an optical axis, is attached, and +A-PLATE, which is a positive uniaxial, optical anisotropic film and parallel to a plane with respect to an optical axis, is attached so that these retardation films are applied to the IPS mode, thereby obtaining more superior characteristics of a viewing angle and a color shift.
As disclosed in Japanese Patent Laid-open Publication No. 2001-166133, a retardation characteristic is controlled through an appropriate method in that a film is made of a polymer is processed to extend, or a refractive index of a thickness direction is controlled so that such a retardation film can be obtained. In particular, the retardation film may be obtained through a method in that a sheet or two sheets of heat-shrinkable films is adhered to a single surface or double surfaces of a film using an adhesive layer or the like, and a contractile force of the heat-shrinkable film is then added to film by means of heat so that the film is extended or contracted in a longitudinal or lateral direction, or both directions under the contractile force.
Further, Japanese Patent Laid-open Publication No. 2000-227520 discloses that assuming that main refractive indexes within a plane be nx and ny, a refractive index of a thickness direction be nz, and nx≧ny, a retardation plate comprises a combination of more than two kinds of retardation films with refractive indexes different from one another, the retardation films each having a refractive index characteristic of nx=ny>nz, nx>ny>nz, nx>ny=nz, nx>nz>ny, nx=nz>ny, nz>nx>ny or nz>nx=ny. In addition, alignment films which are made of various liquid crystal polymers such as a discotic or nematic system, of which alignment layer is supported by a film substrate, or the likes are used as the retardation film.
Further, Japanese Patent Laid-open Publication No. 2003-149441 discloses a retardation film wherein C-PLATE, which is a homeotropic alignment liquid crystal film formed from a homeotropic alignment liquid crystal composition containing a homeotropic alignment side-chain liquid crystal polymer or a correspondent side-chain liquid crystal polymer, and a photopolymerizable liquid crystal compound, and A-PLATE, which is an oriented film with a function of retardation, are laminated and integrated so that a retardation of a thickness direction can be widely controlled.
In this Japanese Patent Laid-open Publication No. 2003-149441, a side-chain liquid crystal polymer, which is a liquid crystal polymer for forming a homeotropic alignment liquid crystal film and comprises a monomer unit (a) containing a liquid crystalline fragment side-chain with a positive refractive index anisotropy and a monomer unit (b) containing a non-liquid crystalline fragment side-chain, is used; the side-chain liquid crystal polymer has the monomer unit (b) containing a non-liquid crystalline fragment side-chain with a alkyl chain and the like except the monomer unit (a) containing a liquid crystalline fragment side-chain included in an ordinary side-chain liquid crystal polymer; and although a hometropic alignment film is not used, a nematic liquid crystal phase is revealed as a liquid crystal state through an operation of a monomer unit containing the non-liquid crystalline fragment side-chain, e.g., through a heat treatment so that a homeotropic alignment can be shown.
However, a retardation film obtained through the methods disclosed in the Japanese Paten Laid-open Publication Nos. 2001-166133 and 2000-227520 has a limitation on its extension to a thickness direction, and there is a problem in that a retardation of the thickness direction cannot be widely controlled. Further, the method of contracting a retardation film by adding the contraction force of a heat-shrinkable film to a film has a problem in that the thickness of the retardation film becomes thick in the range of about 50 to 100 μm so that requirement of reduction in thickness is not sufficiently met.
Further, the method of forming a homeotropic alignment liquid crystal film using a side-chain liquid crystal polymer as disclosed in the Japanese Patent Laid-open Publication No. 2003-149441 has problems in that since a glass transition temperature (Tg), which is a heat treatment temperature allowing the homeotropic alignment liquid crystal film to have a liquid crystal phase, is usually in the range of 60 to 300° C., more specifically in the range of 70 to 200° C., there is required a high temperature heat treatment process; that since time consumed in the heat treatment is also in the range of 20 seconds to 30 minutes, it is difficult to apply the method to a high-speed continuous process; that since there is required a cooling operation of air-cooling, water-cooling or the like to fix the alignment of the homeotropic alignment liquid crystal film after the heat treatment has been finished, it is difficult to apply the method to a continuous process; and that a liquid crystal polymer compound containing a liquid crystal polymer has a low solubility of a compatible solvent.